Fast, high-voltage (HV) pulse generators with output voltages in the 100 kV to 2 MV range are required for accelerating electrons in electron guns or heavy ions in particle accelerators and for relativistic klystron applications. High energy, fast pulse electron guns find applications in e-beam pumped gas lasers and in x-ray generators, the latter being useful for preionizing discharge pumped excimer lasers, as well as for x-ray illumination of materials and for high speed diagnostics, such as x-ray shadowgraph techniques. Other uses for fast HV pulsers are in impulse voltage testers, such as lightning simulators, and for prespiking discharge pumped high energy gas lasers.
In many of these applications, a major part of the electric pulse energy must be deposited into a dynamically varying load, which is typically between 100-1000 ohms of diode impedance for high power electron guns and x-ray generators. The load impedance can drop significantly below these values, however, as the vacuum diode proceeds to close. Gas laser prespikers may have to drive the discharge or load impedance down to approximately one ohm. Successful high voltage pulse generators for these applications should therefore have output impedances in the range of 10-100 ohms and in some instances much lower. Rise times generally should be less than 50 nanoseconds, and pulse durations in the range of 100-500 nanoseconds.
This required combination of extremely high output voltage, low output impedance and fast rise time places severe demands on the type of pulser that can be used, and particularly on the high voltage switches employed in the generating network. Traditionally, these demands have been met with multi-stage Marx banks, switched by triggered spark gaps, as described by E. Marx, Elektrotech. Z. 46 (1925) 1298, by R. A. Fitch, Marx and Marx-Like High Voltage Generators, IEEE Trans. Nucl. Science NS-18 (4th Symp. on Engin. Probl. of Fusion Research 1971) 190, and by F.B.A. Fruengel, High Speed Pulse Technology, Vol. 1, Academic Press (NY 1965) p. 298. Spark gap switches are capable of carrying high currents and providing very fast current rates of rise (dI/dt), but they are generally unsuited for pulse rate frequencies (PRF) above 10 Hz. For high PRF applications, blast spark gaps involving high gas consumption and back-lighted thyratrons (BLT) have been suggested. See F.B.A. Fruengel, High Speed Pulse Technology, Vol. 1, Academic Press (NY 1965), M. Gunderson et al., The Back-Lighted Thyratron, Optic News (Dec. 1989) 37, and W. Hartmann et al., J. Appl. Phys. 65 (1989) 4388. BLTs are disadvantageous in that they are relatively complex, require the separate generation of intense light pulses, and would prove quite expensive for a large number of voltage multiplication stages.
For generating high voltage pulses at high PRF, the conventional hot cathode high voltage thyratron is presently the most reliable switch and has been the workhorse in high voltage pulse applications involving high PRF. Thyratrons are disadvantageous, however, in that they are generally limited to 40 kilovolts of anode voltage and are not capable of generating fast rising, short pulses because of dI/dt limitations. Additionally, thyratrons cannot easily be used in multi-stage voltage multipliers because their cathodes are designed to operate near ground potential, unless the power supplies for controlling cathode and hydrogen reservoir temperatures are isolated by cumbersome high voltage isolation transformers.
A step-up pulse transformer having a secondary-to-primary winding ratio of 30:1 to produce 1 MV, for example, may be used in combination with a thyratron to generate very high voltages at high PRF. This approach, however, is only practical if slow rise times and high output impedances can be tolerated. For example, given an optimistic primary impedance of 5 ohms, including the effects of transformer leakage inductance, the output impedance of such a pulser will be approximately 5 kohms, as determined by the expression Z.sub.s =(N.sub.s /N.sub.p).sup.2. Z.sub.p.
Fast magnetic switches have recently been developed as the result of the availability of metallic glass magnetic cores having extremely rectangular hysteresis loops. Using these metallic glass magnetic cores, it is possible to design saturable core inductors having an impedance which can decrease by more than two orders of magnitude over a time interval of 10 nanoseconds. See C. H. Smith et al., Dynamic Magnetization of Metallic Glasses, Proc. 5th IEEE International Pulsed Power Conf., Crystal City, Va. (1985) 664 and Proc. 18th Power Mod. Symp., Hilton Head, S.C. (1988) 336.
The application of these fast magnetic switches for modulating the electric discharge pulse in gas lasers is described in U.S. Pat. Nos. 4,275,317 and 4,698,518.
At the present time, high voltage multiplier networks having more than two stages, such as Marx banks and multi-state LC inversion multipliers, are limited to low PRFs, because the spark gaps, traditionally used in such devices, exhibit a long recovery time and relatively short electrode life. Conversely, long-life, high-PRF switches, such as thyratrons, are not easily adapted to multiple stage multipliers, because their cathodes must be operated near ground potential.
It is therefore a principal object of the present invention to provide multistage LC inversion networks and Marx banks that employ multiple winding saturable core inductors or transformers as switches. High voltage multipliers so constructed provide the desirable properties of fast switching action and long voltage hold-off periods that are required for many pulsed power applications, while avoiding the drawbacks of the prior art described above. The multiple winding magnetic switches of the present invention exhibit low output impedance and can easily generate fast megavolt pulses at PRFs in the kHz range.